Can disable package installation

Fixed #16

I have the same need as describe in the issue but I just don't want dotdeb packages on my server.

With this new attribute it's easy to skip the repository installation.
I'm wondering if it would be more flexible to include all 3 independently into default, then, perhaps, everyone will be happy, i.e. the ones with cookbook wrapper and the ones who just need the easy way to skip repositories, what do you think?